Each caused a blast equivalent to that released by a nuclear warhead, but at least one was many times as powerful as the bomb that destroyed Hiroshima in 1945.

The explosions were recorded by the Nuclear Test Ban Treaty Organisation which operates a global network of sensors listening out for nuclear weapon detonations.

Most occurred too high in the atmosphere to cause any serious damage on the ground.

But the evidence is a reminder of how vulnerable the Earth is to the threat from space, scientists say.

Video:Nova Scotia Meteor: March 2014

None of the asteroids were detected or tracked in advance by any existing space or Earth-based observatory.

Former astronaut Ed Lu explained his fears as he revealed the new data at a press briefing at the Museum of Flight in the US city of Seattle.

He said: "While most large asteroids with the potential to destroy an entire country or continent have been detected, less than 10,000 of the more than a million dangerous asteroids with the potential to destroy an entire major metropolitan area have been found by all existing space or terrestrially-operated observatories."

Dr Lu is co-founder and chief executive of the B612 Foundation, a research body dedicated to finding ways of protecting the Earth from dangerous asteroids.

The most dramatic asteroid blast in recent times occurred when an object exploded over Tunguska, Siberia, in 1908 with an energy yield equivalent to between 5,000 and 15,000 kilotons of TNT.

Hiroshima was equivalent to 15 kilotons of TNT.

An area of remote forest covering 770 square miles was flattened by the blast.

Video:US Fireball: December 2013

In 2013, a 600 kiloton meteor explosion above the Russian town of Chelyabinsk caused extensive damage to property.

Asteroid impacts greater than 20 kilotons occurred in South Sulawesi, Indonesia, in 2009, the Southern Ocean in 2004, and the Mediterranean Sea in 2002.

Just two days ago, Russian media reported that another fireball had been caught on camera exploding over the city of Murmansk.

In 2018, the B612 Foundation plans to launch the world's first privately-funded deep space mission, Sentinel, which will use an infrared space telescope to identify threatening objects when they are still millions of miles away.

Scientists with the European Space Agency are also planning to launch a probe called Don Quijote which will test whether an asteroid or comet can be deflected.